Quasi-entropy by log-determinant covariance matrix and application to liquid crystals

A quasi-entropy is constructed for tensors averaged by a density function on SO(3) using the log-determinant of a covariance matrix. It serves as a substitution of the entropy for tensors derived from a constrained minimization that involves integrals. The quasi-entropy is an elementary function that possesses the essential properties of the original entropy. It constrains the covariance matrix to be positive definite, is strictly convex, and is invariant under rotations. Moreover, when reduced by symmetries, it keeps the vanishing tensors of the symmetry zero. Explicit expressions are provided for axial symmetries up to four-fold, as well as tetrahedral and octahedral symmetries. The quasi-entropy is utilized to discuss phase transitions in several systems. The results are consistent with using the original entropy. Besides, some novel results are presented.